In coating to steel plates and plastics of automobiles, home electric appliances and the like, for the purpose of shortening of coating process and energy saving, there have been widely adopted a coating method called a wet-on-wet system, or a two-coat one-bake (occasionally three-coat one-bake, for example) system. In these coating systems, for the purpose of VOC (volatile organic compound) reduction at the same time, for paints used in a base layer and an intermediate coating layer, a low VOC paint, for example, an aqueous paint is often used.
However, since paints used in a base layer and an intermediate coating layer have not reached the level, of two-component reaction-type coated layers in terms of coated film performance, there occur problems such as poor adhesiveness, solvent resistance and water resistance, so methods for improving these drawbacks have been required.
In Patent Documents 1 and 2, there is described a method where a paint composed of a resin containing hydroxyl and carboxyl groups and a catalyst is coated as a base coat, next, a clear coat consisting mainly of a hydroxyl group-containing resin and a polyisocyanate compound is coated so that the isocyanate compound in the clear coat paint is partially penetrated into the base coat for hardening.
Patent Document 3 describes that aiming at a similar effect of partially transferring an isocyanate compound in a clear coat into a base coat, as a hardener of the clear coat, a polyisocyanate containing 1 weight % or more, 70 weight % or less of dimer (also called uretdione) of 1,6-hexamethylene diisocyanate (hereinafter, also referred to as HDI) is used in a range of 1.2 to 3.0 in NCO/OH ratio.